Polymers which are to serve as flocculants or viscosifiers tend to provide a high aqueous solution viscosity with the result that it is not practicable to supply such polymers to the user in the form of aqueous compositions. Usually the polymers are supplied as powders or reverse phase emulsions.
The powders are usually made by gel polymerisation followed by comminution and drying or by reverse phase bead polymerisation followed by drying, filtration and, if desired, comminution. The reverse phase emulsions are made by reverse phase emulsion polymerisation to form an emulsion of small (below 10 .mu.m) aqueous polymer droplets in a non-aqueous liquid. Frequently, the user is supplied with this emulsion of aqueous polymer particles. It is, also, known to subject the emulsion to distillation so as to form an emulsion of substantially non-aqueous polymer particles in the non-aqueous liquid, and then supply the user with this. However it is also known to recover powder from the emulsion.
A typical process for this recovery is described in U.S. Pat. No. 3,284,393 and involves diluting the emulsion of aqueous polymer particles with non-aqueous liquid and then flocculating the particles with acetone, filtering them from the emulsion and drying under vacuum.
However this has not proved to be commercially successful. One of the reasons for this is probably that it is difficult to control the agglomeration or precipitation of the emulsion so as to form aggregates of controlled size in an economic manner.
Another way which has been proposed in the literature for recovering water soluble dry, polymer from a reverse phase polymer emulsion is by spray drying. An example of such a disclosure is in U.S. Pat. No. 4,035,317. However spray drying processes also have not proved to be of any significant technical value in the recovery of powder from reverse phase emulsions and, so far as we are aware, there is no commercial production of spray dried granules of water soluble polymer made by reverse phase emulsion polymerisation.
Instead, the present state of the technology is that viscosifier or flocculant polymers which are required in powder form are made by gel or bead polymerisation and they are only made by reverse phase emulsion polymerisation if they are going to be supplied in the form of reverse phase emulsions.
One disadvantage of putting the polymers into powder form by any of these techniques is that it is necessary to put them into the form of a homogeneous aqueous composition before use as viscosifier or flocculant. Thus the water soluble powder is dissolved in water to make a homogeneous solution. It is well known that the contact between particles of water soluble high molecular weight polymer in the presence of water is liable to lead to the formation of gelatinous agglomerates which can take a long time to dissolve. Accordingly special techniques and apparatus have been developed for minimising the accidental formation of these agglomerates during the dissolution of the soluble polymer. As a result, provided that reasonable care is taken, the soluble polymer can be put into true homogeneous solution irrespective of whether it is provided as particles of regular or irregular size and irrespective of whether the particles have been made by gel polymerisation, bead polymerisation or some other technique.
When a water soluble, reverse phase emulsion, polymer is being used in liquid form, the normal process for "activating" the emulsion comprises mixing the reverse phase emulsion with water in the presence of an oil-in-water emulsifier. Again, suitable procedures for achieving a homogeneous solution by such activation procedures are well known and can result in rapid activation of the individual polymer particles, substantially without any permanent or semi-permanent agglomeration of the particles.
Particular problems, however, arise when the polymer has been made in the presence of polyethylenically unsaturated cross linking agent. If the amount of this is very high, then the product is wholly insoluble. However if the amount of cross linking agent is low or medium then the polymer will contain both soluble and insoluble fractions. As an example, it has been known for many years to supply print paste thickeners which are reverse phase emulsions of acrylic polymer cross linked by, for instance, 500 parts methylene bis acrylamide (MBA) or other polyethylenically unsaturated cross linker. Thus the material which is supplied to the user consists of a continuous, non-aqueous, phase in which are dispersed the primary particles, which are particles below 10 .mu.m and each of which is a cross linked acrylic polymer. Thus each primary particle consists of an insoluble cross linked network having the defined particle size throughout which soluble polymer may be distributed. Upon activating this emulsion in water, the soluble polymer migrates out of the particles leaving the insoluble network particles as a suspension in an aqueous solution of the soluble component of the particles.
It is intended that the insoluble components should not agglomerate but should, instead, remain as individual primary particles having a size no bigger than the initial primary particle size, i.e., substantially all below 10 .mu.m and often substantially all below 3 .mu.m. It is necessary that the primary particles do not agglomerate either within the emulsion or during activation since, if they do, they are liable to form gelatinous lumps which will not pass through the printing screen but will instead block it. Occasionally we do see some tendency for the formation of permanent or semi-permanent agglomerates to occur within the emulsion or during activation when the primary particles in the emulsion are below 10 .mu.m in size. Examination of a deposit shows that it consists of a permanent or semi-permanent agglomerate of the primary particles, this agglomerate presumably being formed as a result of the wet primary particles sticking to one another.
Agglomeration in this manner of primary particles which are wet and which contain both soluble and insoluble components is not unexpected in view of the known tendency for soluble polymers to agglomerate. However whereas soluble polymers, once agglomerated, will eventually redissolve, agglomerates formed from soluble and insoluble components will be much more difficult to redissolve and may never redissolve, because of the adhesion between the insoluble particles and the soluble components that are distributed within them in the initial emulsion. It is for this reason that care has to be taken in developing the activation procedures for print paste thickeners so as to minimise the risk of unwanted agglomeration of the primary particles.
There are various other processes where it is known to be desirable to supply the user with primary particles having a size below 10 .mu.m and which have both soluble and insoluble components and wherein the polymer is available during use in the form of particles having a size substantially all below 10 .mu.m. The polymers are made by reverse phase polymerisation of a water soluble ethylenically unsaturated monomer or monomer blend and a low to medium amount of polyethylenically unsaturated cross linking agent to form a reverse phase emulsion of primary particles which contain an insoluble polymer network and a soluble polymer and they are used for various viscosifying and flocculation purposes. Preferred flocculation processes are described in EP 202,780.
In practice such polymers are always supplied to the user in the form of a reverse phase emulsion. This is because the theoretical possibility of providing them in powder form has proved difficult to achieve in a commercially economic and satisfactory manner. For instance we describe in EP 326,382 a process in which a cationic, moderately cross linked, reverse phase emulsion polymer in accordance with EP 202,780 is first dried by distillation to give an emulsion (or dispersion) of dry polymer particles in non-aqueous liquid. These dry polymer particles are then precipitated from the dispersion by adding acetone. They are then dried while entrained in air and are sprayed with water containing fumed silica so as to coat the particles and aggregate them, and they are then further dried in a fluidised bed. Thus this technique first involves drying the particles, then coating and rewetting and aggregating the particles, and then redrying them. Although it is operable it tends to be inconvenient and it is difficult to achieve controlled particle size by this technique. It is noted in EP 326,382 that if the powder, without the silica aggregation step, is added direct to water then serious gel blocking occurs. This again is consistent with the expectation that undesirable permanent aggomeration will occur when the primary particles of soluble and insoluble polymer contact one another while wet.
Many users would prefer to receive a solid polymeric composition consisting substantially only of the relevant polymer, rather than to receive a reverse phase emulsion. This need can be met, with soluble polymers, by supplying a conventional gel or bead product but cannot be met in respect of those compositions where the primary particles have to be below 10 .mu.m and are cross linked and so contain soluble and insoluble components. In particular, it would be desirable to be able to provide powdered products which can be made in an easy and economic manner and which will revert substantially completely to particles upon addition to water such that the performance of the original emulsion polymer is retained. Thus the permanent agglomeration of the wet primary particles has to be avoided.